Can I set up an account for users to forward mail to for learning?

Not in any easy way that works for most mail clients.

The basic problem is that the forwarded message won't be the same as the original message. This is especially true of the message headers, and SpamAssassin's Bayes engine does learn from headers. Most forwarding mechanisms also modify the body.

If your users have a mail client that can forward a copy of the verbatim original message, including headers, as an attachment you might be in luck. In theory you can create an automated account that would strip the attachments off and feed those to sa-learn.

Also some mail clients have a redirect or bounce feature, and some of those will actually send the message along without any significant modification. You'll have to test your specific mail client to be sure.

However, none of this is just as quick and simple as setting up a quick procmail rule and having your users just forward messages.